Methods of producing magnetic materials and to the magnetic materials so produced



w 1 Am 9h 028 3% mm s 3 u M m m c I T m G mm Fm April 17, 1962 4 METHODSOF PRODUCI TO THE MAGNETIC MATERIALS so PRODUCED Filed NOV- 14, 1958 R mN w N I LEVI TUL vi 0 Apr-1117, 1962 F. LEVI 3,029,496

METHODS OF PRODUCING MAGNETIC MATERIALS AND TO THE MAGNETIC MATERIALS SOPRODUCED Filed NOV'. 14, 1958 v 3 Sheets-Sheet 2 INVENTOR 1 g- 7w; viaLEVI United States Patent The present invention relates to methods ofproducing permanent magnet materials and to the permanent magnetmaterialsso produced.

The most widely used permanent magnets today are produced from eastalloys used under various trade names such as Alnico, Alcomax, Ticonal.These permanent magnets are hard and brittle, and contain substantialpercentages of scarce and expensive materials like cobalt and nickel.

Reasonably ductile permanent magnets are also available; these areparticularly useful in instrumentation work, where magnets having a highcoercive force and very small dimensions are often required. Permanentmagnets of this class are, for example, those sold under the trade namesof Cunife and Vicalloy. Cunife contains about 20% nickel and Vicalloyabout 50% of cobalt. Typical compositions are: 20% iron, 20% nickel, 60%copper for Cunife and 34% iron, 53% cobalt, 14% vanadium for Vicalloy.Theoptimum magnetic properties of these materials can bevaried onlyslightly, owing to metallurgical reasons. The best magnetic propertiesare obtained after severe mechanical elongation of the materials, e.g.by means of drawing, and are found along the direction in which thematerials have been elongated. r

The permanent magnetic properties of all the foregoing materials areobtained by a process which includes, essentiallyza solid solution heattreatment, a quench, and an ageing heat treatment. As a result of thisprocess, the final material contains extremely fine particles of a mainferromagnetic phase dispersed within a nonferromagnetic or secondaryferromagnetic phase.

According to a now widely accepted theory of permanent magnetism, thisfinely dispersed state is responsible for the high coerciveforces-varying, broadly, from 400 to 800 oersteds-of the abovematerials.

Theory (see, e.g.: Physical Theory of Ferromagnetic Domains CharlesKittelReviews of Modern Physics, volume 21, No. 4, October 1949, pp.541-583) also suggests that a body containing sufficiently smallparticles of a ferromagnetic material, having negligible coercive forcein the bulk state like iron and cobalt, for ex ample, separated one fromanother by a non-ferromagnetic material ought to be a permanent magnet,or, in other words, possess substantial coercive force. This conditionis achieved with approximately spherical particles of iron when thediameter of each particle is less than about 0.1 of a micron (1 microncm.). Each particle of iron is then considered to contain only onemagnetic domain and is commonly called a singledomain particle. Asingle-domain particle is known as one having uniform magnetization inzero field and the references to a single-domain particle hereinafterappearing are to be interpreted accordingly. The critical diameter belowwhich'a spherical particle is a singledomain particle generally varieswith the material of the particle.

. Further development of the theory has led to the conclusion that thehighest values of coercive force, and generally otherdesirable magneticproperties, should be particles-of iron or of cobalt for example-ofequal round cross-sections each having a diameter somewhat smaller thanthe critical diameter of a single-domain sphere of the same material,all particles being evenly spaced fromeach other, and all particleshaving parallel elongated axes. It is also considered that elongatedparticles of a given material can be single-domain when their diameternormal to the elongated axis is considerably larger than the criticaldiameter of a spherical particle of the same material.

Another important theoretical result is that the in 7 ,I-l is theintrinsic coercive force of an isolated singledomain particle. Itfollows from this relation, that the magnetic properties of the body canbe modified by changing the spacing between the particles.

Most of these theoretical conclusions have been known for over ten yearsand a large amount of experimental work has been carried out in anendeavour to produce bodies having the required structure. This interestderives not only from the fact that it appears possible to producepowerful permanent magnets using only cheap and freely availablematerials like iron, but also because found in a body composed ofelongated ferromagnetic the predicted maximum values of the coerciveforce are much higher than those obtained so far in the above mentionedmaterials Alnico, Cunife and Vicalloy. Although the predictedtheoretical maximum values for 1-1 of isolated elongated particles ofiron having a cross-sectional diameter of about .02 of a micron and alength exceeding about .1 of a micron, vary between about 2500 and10,000 oersteds (see, e.g.: Kittel, already quoted above and:Reproducing the Properties of Alnico Permanent Magnet Alloys withElongated Single-Domain Cobalt-Iron Particles Lubersky, Mendelsohn,Paine; Conference on Magnetism and Magnetic Materials, Boston,1956published February 1957, pp. 133-144) properly compacted particlespossessing even the lowest predicted maximum Value would result in apermanent magnet more powerful than any magnet commercially producedtoday.

To the best of the applicants knowledge, all previous methods ofproducing these composite bodies rely on the compacting of previouslymanufactured fine particles of ferromagnetic material. The resultingbodies are mechanically soft, but are not ductile and cannot be drawn orrolled to small dimensions. 1

Some of the methods used for the manufacture of the particles are: thedecomposition of organic salts of iron or of iron and cobalt withsubsequent hydrogende-oxidation; electro-deposition of iron or of ironand cobalt on a mercury cathode, either moving or stationary, followedby removal of the mercury; casting alloys of iron or of iron and cobaltwith aluminium under suitable conditions followed by chemical removal ofthe aluminium.

Many procedures have also been suggested and used to' protect the fineparticles from oxidation, since the ma: terials become pyrophoric whensubdivided to the required degree, and from coalescence duringcompacting.

It is obvious that, with all hitherto known processes, the shapes anddimensions of the particles are not uniform and it is extremelydifficult to obtain a large percentage of particles having optimumproperties. Furthermore the alignment of the particles, which is usuallyattempted by applying a magnetic field during some of the stages leading to, and including, the final compacting and eventual heat treatment,is only partially successful owing to frictional forces, to the presenceof particles which are not Pafented Apr. 17, 1962 susceptible toalignment and even to the permanent magnetic properties of the particlesthemselves which cause formation of particle clusters.

All the above difiiculties are best appreciated when it is rememberedthat iron or cobalt powders having the desired properties are invisiblewith the best optical microscope, since they have cross-sectionaldimensions smaller than the wave length of light, and are comparable tosmoke particles. When all these factors are taken into consideration,the results achieved with know methods are indeed remarkable and a clearindication of the probable correctness of the theory.

The primary object of the present invention is to provide a novel methodof preparing permanent magnet materials containing very finely dispersedelements, which substantially avoids the above mentioned dilficulties.

According to the invention materials possessing desirable permanentmagnet properties are produced by reducing the cross-section of acomposite body containing multidomain ferromagnetic elements, separatedfrom each other by suitable materials, until the ferromagnetic elementsbecome single-domain owing to their reduced cross-sections.

For this purpose a starting material is employed which comprises acomposite body containing a plurality of ferromagnetic elements each ofan appropriate shape in cross-section and of a size in cross sectionsubstantially larger than that of the particles finally desired and eachseparated by a non-ferromagnetic material or a ferromagnetic materialdifferent from that of the ferromagnetic elements. The ferromagneticelements may be separated from each other by more than one kind ofmaterial provided the material is a non-ferromagnetic material or is aferromagnetic material different to that of the ferromagnetic elements.The composite body is then subjected to an elongation process by meansof drawing, rolling, swaging or other similar techniques, until theferromagnetic elements within the body each have a sufficiently smallcross section to be single domain. It Will be appreciated that, in thismanner, elongation and alignment of all the elements is automaticallyachieved. The body containing the lements can be of substantial crosssection for economy and ease of operation. When a given body becomes toosmall in cross section and further reduction is necessary, the body maybe cut into shorter lengths which are then assembled to form a secondcomposite body. The process may then be repeated.

The ferromagnetic elements may have substantially equal and likeconfigurations in cross-section and may be substantially equispaced fromeach other in planes normal to the elongation direction.

Each ferromagnetic element may consist of a rod or wire which is encasedin a sleeve of non-ferromagnetic material or a ferromagnetic materialdifferent to that of the rod or Wire. The elements may each be subjectedto elongation with or without heat treatment prior to assembly in thecomposite body.

During the elongation process the composite body may be annealed orotherwise heat treated at an appropriate temperature and for anappropriate period depending upon the materials forming the body andproperties required for the final magnetic material. At no stage,however, should the temperature be sufficiently high to bring theelements into solid solution.

Although the method is not limited to the manufacture of bodies having ahigh coercive force as the only desirable magnetic property, theapplication of the method to this particular aim will now be describedin detail.

As previously explained, a substantial coercive force can theoreticallybe obtained in a composite body containing parallel and equispaced ironfilaments having round cross sections of about 0.1 of a micron diameterand lengths exceeding 0.5 of a micron.

In one practical embodiment of the invention a magnetic body wasproduced containing iron filaments estimated to satisfy the aboveconditions and having an approximate intrinsic coercive force ofoersteds in the hard drawn condition and of 200 oersteds afterannealing. Although much higher coercive forces have been achieved inother embodiments, some of which are hereinafter described, themicrophotographs relating to Example I are the most suitable for a clearunderstanding of the principles of the invention.

Example I A tube of 5% tin bronze with 0.024" wall thickness was drawnover a 0.070" diameter iron wire. The resulting composite wire was drawnfrom the diameter of to a diameter of .005". The wire was annealed atapproximately 420 C. for half an hour at stages corresponding to about75% area reduction.

A bundle of 300 substantially parallel composite wires 0.005" diameterformed as above described was inserted in a 5% tin bronze tube with0.014" Wall thickness to form a first compact. The starting diameter ofthis first compact, after the outer tube had been drawn over the wiresfairly tightly, was about 0.125". This first compact was drawn to 0.005"diameter. The compact was annealed at approximately 420 C. for half anhour at stages corresponding to about 50% area reduction.

FIGURE 1 shows a drawing of a microphotograph of a cross-section of thisfirst compact at 0.030 diameter containing 300 iron filaments each ofapproximately 20 microns diameter.

A bundle of 300 substantially parallel first compacts of 0.005 diameterwas inserted in a 5% tin bronze tube with 0.014" wall thickness to forma second compact. The starting diameter of this second compact, afterthe outer tube had been drawn over the bundle of first compacts fairlytightly, was about 0.125". This second compact was drawn to 0.005"diameter. The second compact was annealed at approximately 420 C. forhalf an hour at stages corresponding to about 50% area reduction.

FIGURES 2, 3 and 4 show drawings of microphotographs of portions of thecross-sections of this second compact at 0.035, 0.015" and 0.005"diameter respectively. The estimated equivalent cross-sectional diameters of the individual iron filaments are respectively approximately:0.7 of a micron, 0.3 of 21 micron and 0.1 of a micron. The individualiron filaments are clearly visible in FIGURE 2 but are blurred in FIGURE3 and indistinguishable in FIGURE 4. This is due to the fact that thediameters of the individual filaments cannot be resolved by opticalmicroscopes when they are smaller than about 0.5 of a micron.

FIGURE 5 shows a drawing of a microphotograph of a completecross-section of the second compact at 0.005" diameter. Each of the 300dark areas contains an estimated 300 iron filaments, each about 0.1 of amicron diameter. The intrinsic coercive force at this stage was, asalready mentioned, about 100 oersteds before anneal and 200 oerstedsafter anneal.

FIGURE 6 shows a drawing of a microphotograph of a longitudinal sectionof a portion of the second compact at 0.025" diameter. The ironfilaments have an estimated equivalent diameter of 0.5 of a micron andcan be seen in the figure running substantially parallel to each otherinside each first compact.

TWo complete first compacts can be seen in FIGURE 6. Although smallmisalignments are unavoidable, it can be seen that most of the filamentsappear to be of uniform cross-section, are of considerable length andare equally spaced from each other.

Two other practical embodiments will now be described in which thespacing between the individual iron filaments corresponds to about 35%iron in Example II and 21% in Example III.

Example 11 was drawn over a 0.070 diameter iron wire. The resultingcomposite wire was drawn to 0.0025 diameter with anneals atapproximately 460 C. for minutes at stages corresponding to about 75%area reduction.

A first compact was made containing 1000 wires 0.0025" diameter,surrounded by a 0.014 wall thickness bronze tube and drawn to 0.00

The first compact was annealed at approximately 460 C., for ten minutesafter a reduction corresponding to about 95% area reduction and atreductions corresponding to about 50% area reduction after the firstanneal.

A second compactwas made containing 800 first compacts 0.003" diametersurrounded by a 0.014" wall thickness bronze tube and drawn to 0.005 Theestimated individual iron filament equivalent diameter was then .05 of amicron.

The second compact was annealed at approximately 460 C. for ten minutesafter a reduction corresponding to about 70% area reduction atreductions corresponding to about 50% area reduction following the firstanneal. The approximate intrinsic coercive force of the second compactat 0.005" was 330 oersteds in the hard drawn state and 420 oerstedsafter anneal. The computed average intrinsic coercive force of theisolated particles after anneal is Example III A tube of 5% tin bronzewith 0.0 Wall thickness was drawn over 0.040" diameter iron wire. Theresulting composite wire was drawn and first and second compacts madesimilarly to Example II. v

When the second compact was drawn to 0.005"- the estimatedindividualiron filament equivalent diameter was 0.04 of a micron.

The iron percentage inside the outer bronze tube in the first compactwas 21%.

The approximate intrinsic coercive forceof the second compact at 0.005".was 480 oersteds in the hard drawn state and 600 oersteds after anneal.

The computed average intrinsic coercive force of the isolated particlesafter anneal is iH =760 oersteds .made according to the invention canexceed the value of 800 oersteds. Whilst this result is suificient toallow the manufacture of ductile materials having high coercive forcesand using only cheap and freely available materials, like iron, a resultwhich is believed to be both useful and novel, itis expected that themethod of the invention is capable of producing magnetic materials withmuch higher coercive forces than those mentioned. In this regard it willbe obvious that there are a great number of combinations of materials,compacting and reducing methods and annealing schedules which arepossible.

Another feature of thematerial of the invention is that thedemagnetizing curve of magnetic materials manufactured as in theexamples previously described have a substantially square demagnetizingcurve if they are subjected to an annealing treatment at temperatures ofbetween 420 C. to 470 C. for about five to twenty minutes. As anexample, FIGURE 8 shows the demagnetizing curves of a material producedas described in Example 11 in the hard drawn and in the annealedconditions, when the estimated iron filament diameter was about 0.07 ofa micron.

The iron wire used in all the embodiments herein described was lowcarbon 0.1 manganese semikilled steel wire as used for the manufactureof nails and similar articles, hydrogen purified.

The 5% tin bronze tubing contained approximately 95% copper, 4.5% tin,plus minor additions of phosphorus, iron and Zinc.

It is believed that a variety of other materials can be used to carryout the-invention. Whilst it is not desired to be limited by anyspecific theory, experience suggests that the materials, temperaturesand frequency of the interstage anneals have to be selected was toachieve a suitable compromise between ductility of the compact and finegrain size of both the ferromagnetic filaments and of the spacingmaterials. Filament breaks result in uneven filament cross-sections;increase of grain size causes deformation of the cross section of theoriginal ferromagnetic elements. Both faults reduce the coercive forceof the compacts.

It should'be noted that the times and temperatures quoted in the aboveexamples, refer to a process where' the composite bodies were placedinside a mufiie containing a neutral atmosphere and the mufile heated inan air circulating oven set at the quoted temperature.

The actual time during which the composite body was annealed at thequoted temperature was somewhat less than the total quoted time owing tothe presence of the muflle.

It has been found that a good guide as to the selection of suitablevalues of the process variables is obtained from an examination of thecross-sections of the ferromagnetic elements under a microscope. Theferromagnetic elements cross-sections can then be checked for shape,size and uniformity of spacing at different stages of the process andthe process modified to obtain the desired result.

Having now described my invention, what I claim as new and desire tosecure by Letters Patent is:

1. A method of producing permanent magnets comprising the steps ofproviding a plurality of rod-like elements of a ductile ferromagneticmetal, enclosing each element in a casing of a different metal havingsubstantially the same ductility assembling a plurality of the encasedferromagnetic elements together to form a composite body, and compactingthe assembly to uniformly elongate the composite body by repeatedcompacting operations until substantially all of the ferromagneticelements are single domain.

2. A permanent magnet comprising a plurality of laterally spacedferromagnetic elements substantially all of which are single domain, anda metallic material different to the material of the elements separatingsaid elements from each other, said permanent magnet being produced bythe method of claim 1.

. 3. The method of producing permanent magnets according to claim 1wherein said composite body is subjected to annealing steps between andafter saidrepeated compacting operations at a temperature less than thatwhich would'bring said ferromagnetic elements into solid solution. a

4. The method of producing permanent magnets according to claim 3wherein said annealing steps are performed in the range of 420-470 C.

5. A method as claimed in claim 1, wherein the composite body issubjected to heat treatment during elongation, the duration andtemperature of the heat treatment and the stage at which the heattreatment is effected being predetermined in accordance with theproperties required for the permanent magnet material.

6. A method as claimed in claim 1, wherein the ferromagnetic elementsand the separating enclosing metal of the casings contained in thecomposite body are selected so that they will have similar hardness andrecrystallization properties throughout the process in order that theymay be uniformly elongated.

7. A method as claimed in claim 1, wherein each ferromagnetic element issubjected to elongation prior to assembly in the composite body.

8. A method of producing permanent magnets comprising the steps ofproviding a plurality of rod-like elements selected from the groupconsisting of ductile ferromagnetic metals and ferromagnetic alloys ofmetal, enclosing each element in a casing of a different metallicmaterial having substantially the same ductility, assembling a pluralityof the encased ferromagnetic elements together to form a composite body,and compacting the assembly to uniformly elongate the composite body byrepeated operations until substantially all of the ferromagneticelements are single domain.

9. A method as claimed in claim 8, wherein the ferromagnetic elementshave substantially equal and like configurations in cross-section andare substantially equispaced from each other in planes normal to theelongation direction.

10. A method as claimed in claim 8, wherein the composite body issubjected to heat treatment during clongation, the duration andtemperature of the heat treatment and the stage at which the heattreatment is effected being predetermined in accordance with theproperties required for the permanent magnet material.

11. A method as claimed in claim 8, wherein the ferromagnetic elementsand the enclosing metallic material are selected so that they will havesimilar hardness and recrystallization properties throughout theproperties throughout the process in order that they may be uniformlyelongated.

12. A method as claimed in claim 8, wherein the ferromagnetic elementsare in the form of wires.

13. A method as claimed in claim 8, wherein each ferromagnetic elementis subjected to elongation prior to assembly in the composite body.

14. A method of producing permanent magnets comprising the steps ofproviding a plurality of rod-like elements of a ductile ferromagneticmetal, enclosing each element in a casing of a dilferent metal havingsubstantially the same ductility, assembling a plurality of the encasedferromagnetic elements together to form a primary composite body, andcompacting the assembly to uniformly elongate the primary compositebody, repeating the preceding steps to form a plurality of elongatedprimary composite bodies; enclosing a plurality of elongated primarycomposite bodies in a casing of a metal ditferent from the metal of theferromagnetic elements and having substantially the same ductility toform a secondary composite body, and compacting the secondary compositebody to uniformly elongate the secondary composite body by repeatedcompacting operations until all of the ferromagnetic elements containedtherein are single domain.

15. A method as claimed in claim 14, wherein each primary composite bodyis encased prior to elongation in at least one sleeve composed of ametal different to that of the ferromagnetic elements but havingsubstantially the same ductility.

16. A method as claimed in claim 14, wherein the secondary compositebody is heat treated after elongation to develop optimum magneticproperties.

17. A method of producing a permanent magnet comprising the steps ofproviding a plurality of iron wires each having a diameter no smfllerthan 10 microns, enclosing each wire in a casing of metal other thaniron having substantially the same ductility, assembling a pinrality ofthe encased iron wires together to form a composite body, and compactingthe composite body by repeated compacting operations until the diameterof each iron wire is less than 1 micron.

'than iron and constructed in accordance with the method of claim 17.

19. A method as claimed in claim 17, wherein the iron wire enclosingmetal is 5% tin bronze.

20. A method as claimed in claim 17, wherein the iron wires havesubstantially equal and like configurations in cross section and aresubstantially 'equispaced from each other in planes normal to theelongation direction.

21. A method as claimed in claim 17, wherein the composite body issubjected to heat treatment during elongation, theduration andtemperature of the heat treatment and the stage at which the heattreatment is eifected being predetermined in accordance with theproperties required for the permanent magnet.

22. A method as claimed in claim 17, wherein the iron wire enclosingmetal is selected so that it will have similar hardness andrecrystallization properties as the iron wires throughout the process.

23. A method as claimed in claim 17, wherein each iron wire is subjectedto compacting for elongation prior to its assembly in the compositebody.

24. A method as claimed in claim 17, wherein the composite body afterelongation is heat treated to develop optimum magnetic properties.

25. A method of producing a permanent magnet comprising the steps ofproviding a plurality of iron wires each having a diameter no smallerthan 10 microns, enclosing each wire in a casing of a metal other thaniron and having substantially the same ductility, assembling a pluralityof the encased iron wires together to form a primary composite body,compacting the assembly to uniformly elongate the primary compositebody; repeating the preceding steps to form a plurality of elongatedprimary composite bodies; enclosing a plurality of elongated primarycomposite bodies in a casing of a metal other than iron and havingsubstantially the same ductility to form a secondary composite body, andcompacting the assembly to uniformly elongate the secondary compositebody by repeated compacting operations until the diameter of each ironwire contained therein is less than 1 micron.

26. A method of producing a permanent magnet comprising the steps ofselecting a plurality of rod-like elements from the group consisting offerromagnetic metals and ferromagnetic alloys of metals, enclosing eachferromagnetic element in a casing of at least one non-ferromagneticmaterial selected from the group consisting of non-ferromagnetic metalsand non-ferromagnetic alloys of metals and having substantially the sameductility as said ferromagnetic elements, assembling a plurality of theencased ferromagnetic elements together to form a composite body andcompacting the assembly to uniformly elongate the composite body untilsubstantially all of the ferromagnetic elements are single domain.

References Cited in the file of this patent UNITED STATES PATENTS1,883,205 Whitehead Oct. 18, 1932 2,234,127 Mautsch Mar. 4, 19412,264,285 Bennett Dec. 2, 194-1 2,682,021 Elmen June 22, 1954 2,717,946Peck Sept. 13, 1955 2,718,049 Prache Sept. 20, 1955 2,880,855 NachtmanApr. 7, 1959 OTHER REFERENCES Kittel: Physical Theory of FerromagneticDomains, Reviews of Modern Physics, volume 21, No. 4, October 1949, Pp.541-583.

1. A METHOD OF PRODUCING PERMANENT MAGNETS COMPRISING THE STEPS OFPROCIDING A PLYRALITY OF ROD-LIKE ELEMENTS OF A DUCTILE FERROMAGNETICMETAL, ENCLOSING EACH ELEMENT IN A CASING OF A DIFFERENT METAL HAVINGSUBSTANTIALLY THE SAME DRY DUCTILLY ASSEMBLING A PLURALITY OF THEENCASED FERROMAGNETIC ELEEMENTS TOGETHER TO FORM A COMPOSITE BODY, ANDCOMPACTING THE ASSEMBLE TO UNIFORMLY ELONGATED THE COMPOSITE BODY BYREPEATED COMPACTING OPERATIONS UNTIL SUBSTANTIALLY ALL OF THEFERROMAGNETIC ELEMENTS ARE SINGLE DOMAIN.
 2. A PERMANENT MAGNETCOMPRISING A PLURALITY OF LATERALLY SPACED FERROMAGNETIC ELEMENTSSUBSTANTIALLY ALL OF WHICH ARE SINGLE DOMAIN, AND A METALLIC MATERIALDIFFERENT TO THE MATERIAL OF THE ELEMENTS SEPARATING SAID ELEMENTS FROMEACH OTHER, SAID PERMANENT BEING PRODUCED BY THE METHOD OF CLAIM 1.